TEHRAN — Iran on Tuesday celebrated the start of the process of loading 163 fuel rods into the core of its first nuclear power plant reactor, putting it within months of operation.

The Bushehr reactor, cast by Tehran as a showcase of its peaceful nuclear intentions, is separate from other more contentious operations elsewhere in the country where Iran is seeking to enrich uranium. But the timing is delicate in diplomatic terms, as tighter sanctions are being put in place against Tehran by the United Nations Security Council, the United States and the European Union.